Resonant converters are a type of direct current to direct current (DC to DC) electric power converter that include a network of inductors and capacitors tuned to resonate at a particular frequency. A resonant converter may need to handle a wide range of input voltages and a wide range of output voltages. FIG. 1A shows an example of a resonant converter.
In this example, the transformer T 102 has a secondary winding 103 with a center tap c connected to drive the load, represented by the resistance RL 109, with the output voltage VO at an output node of the DC to DC converter, where VO is used to represent both the output node and the voltage level at that node. The center tap c is also connected to ground through the capacitor 105. The upper and lower taps of the secondary winding are connected to ground though the diode D1 138 on the one end and through the diode D2 139 on the other. The diodes D1 138 and D2 139 can also be replaced with actively controlled MOSFETs or other switches.
On the primary side, the LLC (inductor-inductor-capacitor) elements of the resonant tank are the inductors Lr 131 and Lm 133 and the capacitor Cr 135 that are connected in series between the node a and ground. The inductor Lm 133 is connected in parallel with the primary winding 101 of the transformer T 102. In this example, the inductor Lm 133 is connected through the capacitor Cr 135 on the one side, and on the other side to the node a through the inductor Lr 131. The switches Q1 121 and Q2 122 are connected between the + and − terminals of a DC input voltage source Vin 107 and are alternately switched on to a generate two-state waveform at the node a. The switches Q1 121 and Q2 122 can be implemented as MOSFETs or other transistors, for example.
FIG. 1B shows the waveform 11 at node a. The output voltage Vo is regulated by varying the frequency of the waveform, which often differs significantly from the resonant frequency of the resonant tank of inductors Lr 131 and Lm 133 and the capacitor Cr 135. For example, in an application as a battery charger for use with an electrical vehicle, the input voltage Vin could vary between 680-800 volts, while the DC output voltage Could be in the 400-750 volt range. In operating over such a wide range of input and output voltages, typical resonant converters are often far from their optimal resonant point, resulting in low efficiency.